1. Field
The present disclosure relates generally to power converters and, in particular but not exclusively, relates to accurately sensing output overvoltage conditions.
2. Description of Related Art
Many electronic devices, such as cell phones, laptop computers, etc., use direct current (dc) power to operate. Conventional wall outlets generally deliver a high voltage alternating current (ac) power that needs to be transformed to dc power in order to be used as a power source by most electronic devices. Switch mode power converters are commonly used due to their high efficiency, small size, and low weight to convert the high voltage ac power to a regulated dc power. In operation, a switch is used to provide a desired output by switching at a variable duty ratio (typically, the ratio of on-time of the switch to total switching period) or by varying the number of ON and OFF cycles of the switch per unit time.
A switch mode power converter may also employ a controller that typically provides output regulation by sensing the output and controlling the switch in a closed loop. The controller may receive a feedback signal representative of the output and then vary one of the control parameters (e.g., duty ratio or the number of ON and OFF cycles of the switch per unit time) based on the feedback signal to regulate the output to a desired quantity.
Some switch mode power converters may have a coupled inductor, also referred to as a transformer, which can be used to both transfer energy and provide galvanic isolation between the input and the output of a power converter. A transformer typically includes two windings: a primary winding coupled to the input of a power converter and a secondary winding coupled to the output of the power converter; but in some cases, the transformer may also include a third winding, which may be referred to as a bias winding, to be used for various different purposes. In one example, a bias winding may be used to supply power to a controller of a power converter. In another example, a bias winding may be used to provide a feedback signal to a controller. In yet another example, a bias winding may be used to detect an overvoltage condition at the output of a power converter.
A controller of a switch mode power converter may have an input terminal coupled to a bias winding to realize multiple functions, such as powering the controller, detecting the overvoltage condition, etc. In one example, a bias winding signal representative of the voltage across the bias winding (e.g., bias winding voltage) may be used to provide a power signal to the input terminal for powering the controller. Since the bias winding voltage may be representative of the output (e.g., the output voltage) of the power converter, the same bias winding signal may also be used for detecting the overvoltage condition from the same input terminal.